Raw sugar is produced by milling sugar cane and is, of necessity, done in close proximity to the fields in which the sugar cane is grown. The raw sugar includes soluble and insoluble impurities which are normally removed by a process called refining which is usually carried out in a sugar refinery. Conventional sugar refining processes are normally, though not necessarily, carried out close to consumer markets and often at great distances from the sugar mill in which the raw sugar was produced. The process conventionally used for sugar refining involves dissolution of the washed raw sugar to produce a melter liquor. The melter liquor is then limed and subjected to a process of carbonatation or phosphatation in which calcium carbonate or calcium phosphate, respectively, is caused to be precipitated and the precipitate is filtered from the liquor in the case of carbonatation and skimmed from the liquor after aeration in the case of phosphatation. Substantially all of the suspended particulate material in the melter liquor is removed with the precipitate during filtration. This procedure, which is generally referred to as clarification, results in a straw coloured clear liquor.
The clear liquor is subjected to decolorisation, typically by being passed through a column containing an adsorbent material such as bone charcoal, resins or granular carbon. The decolorisation process results in a fine liquor from which white sugar is crystallised in vacuum pans. The resulting suspension of crystals in syrup is separated in centrifugals and the sugar is then dried to produce free flowing white crystal sugar.
Australian Patent Specification 84859/91 describes an alternative process for refining cane raw sugar. In this process raw sugar is dissolved to produce a melter liquor and then boiled to crystallise the sugar in the melter liquor. This sugar is recovered as a very low colour intermediate sugar (VLC sugar). If redissolved and filtered the VLC sugar yields a clear fine liquor. When boiled this fine liquor produces white sugar crystals of a colour equivalent to the white refined sugar produced in a conventional sugar refinery employing a traditional decolorising system such as char, granulated carbon or an ion exchange resin.
White sugar should have a turbidity value of approximately 5 or less to be satisfactory in commercial markets. Experience with a variety of raw sugars has shown that the process described in Australian Patent Specification 84859/91 could not consistently meet this specification.